This invention relates to solenoids, and particularly to damping arrangements for such solenoids.
There are a number of applications for slow-acting electromagnetic actuators which at the present time can be satisfied only by devices that are inordinately expensive for the results desired. In the magnetic disc and drum recording art, for example, it is usually desired to start the high inertia disc or drum and bring it slowly up to speed with the delicate transducing heads lifted or withdrawn from the recording surface, and then to lower the heads to the surface very gradually so as to avoid impact or "crash" damage to both head and surface. Solenoids have been used, as exemplified by U.S. Pat. Nos. 3,180,943 and 3,005,676, but only in environments where an air bearing can be expected to form and cushion the approach of the head to the recording surface, or alternatively, only in conjunction with cumbersome and space-filling damping means for slowing the solenoid action; or by the use of expensive timing motors (U.S. Pat. Nos. 3,855,623 and 3,870,835), the cost of which is out of proportion to the importance of the function performed. The same objections apply to prior art equipment of commercial availability in the magnetic tape-lifting art; that is to say, in the art of lifting magnetic tape away from delicate transducing heads during fast forward or rewind operation of the tape reels so as to avoid undue abrasion and wear of the head surfaces, and of the tape itself.
In the solenoid art, ways have been found to slow the plunger action. For example, external dash pots have been coupled to such solenoids, but at the expense of using valuable space that might have been much better devoted to other uses. Likewise, solenoids have been devised with restricted orifices for the plunger cavity; see for example those produced by the Regdon Corporation of Brookfield, Illinois. However, the slowest times obtainable in such devices are in the order of fifths of a second or less, whereas in the head-lifting and tape-lifting arts, periods many times longer are needed to avoid damage to the heads and recording elements.
Accordingly, it is an object of this invention to provide a damped linear solenoid of the restricted-orifice type, having improved time-delay operating characteristics.